Industrial geometric measuring is carried out today using a number of different methods. The most used optical methods are:
use of theodolites; PA1 photogrammetry, i.e., camera-based measuring where the cameras conventionally are based on photographic film, or more recently, are based on electronic sensors; PA1 use of laser rangefinders, where these may be based on the modulation of light and the detection of the phase of the returned beam, or on laser interferometry. PA1 It is difficult to steer the laser beam so that it always strikes the reflector unit. It is particularly difficult to find the beam again if it is "lost". PA1 Laser trackers based on interferometry are in general relative, i.e., they measure difference in distance to a reference point by counting interference rings. If the beam is broken, control of the number of interference rings is lost, and the process must be started again from the reference point. PA1 It is only the position of the reflector unit that is determined. The orientation of the reflector unit remains unknown. In order to be able to measure a selected point on an object directly, the reflector unit must be provided with a contact point at a known distance from the centre thereof. The point must be measured repeatedly whilst the reflector unit is moved around the contact point when this is held still. PA1 Accurate determination of direction requires complex and very precise mechanical solutions, and also compensation for variations in temperature and mechanical operation. PA1 There is dependence upon clear line-of-sight between laser tracker and measuring point. PA1 high accuracy through the combination of accurate direction information from the camera and accurate distance information from the laser rangefinder; PA1 access to hidden points, and also geometric entities such as holes or cylinders so that these can be measured; PA1 simplified design of the laser, in that the camera registers the direction to the reflector unit.
Modern photogrammetry systems, also known as videogrammetry systems, are based on video camera techniques. These register the position of points in the form of active light sources, reflecting points or characteristics of the object to be measured (e.g., holes). The points may be registered simultaneously by two or more cameras, or they can be imaged sequentially from a number of different camera positions. The spatial position of the points is calculated using mathematical methods which include the automatic determination of the position and orientation of the cameras for each individual image, and also correction for the cameras' lens errors and other factors which produce a non-ideal image. The cameras may also be pre-calibrated, i.e., correction of the image points is based on a calibration table or other mathematical correction.
Modern photogrammetry systems are marketed by the Norwegian company Metronor AS, the Swiss company Imetric SA and the US company GSI (Geodetic Services Inc.) Metronor's system is described in Norwegian Patents Nos. 164 946, 165 046, 169 799, 174 025, and also Norwegian Patent Application No. 931873.
Metronor's system is based on pre-calibrated cameras. The system is optimized in order to determine the position of active light sources. A measuring tool known as a light pen is used to mark the points that are to be measured. The light pen has a minimum of three light sources in known positions relative to its contact point. The coordinates of the contact point can be determined by simultaneously taking the image of the light sources.
Imetric and GSI offer systems where the cameras are not pre-calibrated, but are calibrated for each individual measuring operation. The cameras register the position of retroreflector targets. These are illuminated by flash lamps mounted on the cameras. The companies have also developed touch tools similar to Metronor's light pen, where the active light sources are replaced by retroreflector targets.
The photogrammetry systems determine directions in space through imaging (projection). The accuracy depends on the quality of the camera, the nature of the points to be measured, and in particular on the geometrical factors. Geometrical factors which influence accuracy are position, density and distribution of measuring points, the number of cameras or images, and position and orientation of the cameras, and also whether the cameras' lens errors are pre-determined.
The chief disadvantage of photogrammetry systems is that a measuring point must be registrable by two cameras simultaneously or in sequence by locating a camera in at least two different positions.
Laser rangefinders based on interferometry are internationally known under the product name "Laser tracker". A laser tracker consists of a laser, a mirror system for controlling the laser, a reflector unit, distance and direction sensors, and a computer. The reflector unit, also known as a "corner cube" or prism reflector, reflects light back parallel to the emitted beam. The laser beam is steered so that it always strikes the reflector unit. This is accomplished in that the laser tracker contains a sensor which detects the striking point on the reflector unit. In general, a laser tracker registers both direction and distance, and hence three-dimensional coordinates of the measuring point. The distance is determined by interferometry. The direction is determined by registering the orientation of the mirrors. The distance measurement exhibits high accuracy, whereas the direction is often determined with less precision.
Laser trackers have major disadvantages:
Leica and Chesapeake Lasers are among the companies producing laser trackers.
Other laser rangefinders are based on the modulation of emitted laser light, and the detection of the phase of the detected light. Fine resolution requires a high modulation frequency to be used. In order to avoid ambiguity when the reflecting point is moved more than one modulation period, several modulation frequencies are used, and the total phase gives the absolute distance. Thus, an absolute rangefinder is obtained.
Since a pure rangefinder only provides information related to the one-dimensional distance to the measuring point, several rangefinders must be combined in order to compute spatial position. Three distances are necessary in order to determine three-dimensional coordinates for one point. Examples of such systems are described in U.S. Pat. No. 5,305,091.
Routine inspection of mechanical structures is often based on measuring a number of fixed control points. This applies, for example, to production fixtures in the aviation industry. The control points may be made in the form of holes of a fixed diameter. Targets for photogrammetry, theodolite measuring or laser trackers are produced for these holes. Routine inspection of the structures involves the regular measurement of these points.
In the present invention it is proposed to combine photogrammetry technology with laser distance and angle measuring, so that the advantages of both methods are used to the full, whilst their disadvantages are avoided. The following is thus achieved: